Process for preparing microcapsules include a coacervation process, an interfacial polymerization process, an internal polymerization process, and an external polymerization process.
The coacervation process, though having been widely employed suffers from the following defects, because gelatin is used therein as an essential component:
(a) microcapsules formed are poor in water resisting property, PA0 (b) the cost for producing microcapsules is high, PA0 (c) it is difficult to produce a concentrated solution of microcapsules, PA0 (d) the process for microencapsulating is complicated, PA0 (1) preparing an aqueous solution containing a water-soluble high polymer; PA0 (2) emulsifying and dispersing a hydrophobic oily liquid, to which a polyisocyanate or a prepolymer therof and a polyhydroxy compound are added in advance, in the aqueous solution prepared in step (1); PA0 (3) adding aqueous solution of melamine and formaldehyde or an initial condensation product (condensate) thereof to the emulsion prepared in step (2); PA0 (4) adjusting the pH of the system to 5.0 to 7.0, preferably 5.5 to 6.4, and raising the temperature of the system to 50.degree. to 80.degree. C., preferably 60.degree. to 70.degree. C. with stirring, thereby forming an inner wall of polyurethaneurea resin from the inside of the oily liquid dispersed and at the same time, forming an outer wall of melamine/formaldehyde resin through polycondensation reaction of melamine and formaldehyde.
and so on.
The interfacial polymerization process consists in using a hydrophobic monomer and a hydrophilic monomer, and causing a polymerization reaction therebetween at the core materials and water interfaces to form film walls. This process also has several disadvantages in that it is restricted in the possible combinations of reacting monomers to be selected from a handling point of view, it is difficult to ensure dense microencapsulation, it is difficult to control the polymerization reaction, it is difficult to thicken the film walls of the microcapsules, and so on.
In the internal polymerization process, wall films are formed by polymerization of reactants from the inside of a core material, as described in German Pat. No. 2,342,066, while in the external polymerization process the wall films are formed by polymerization of reactants from the outside of a core material, as described in U.S. Pat. Nos. 4,233,178, 4,251,386, 4,356,109, 4,219,604, 4,221,710, and Japanese Patent Application (OPI) No. 51238/81 (the term "OPI" as used herein means an "unexamined published application").
With the recent expansion of fields to which microcapsules are applicable, the following requirements have come to be applied to processes for producing microcapsules:
(1) they can be applied to various kinds of core materials,
(2) they enable encapsulation in a high concentration and with high yield,
(3) microencapsulation can be carried out at a low cost,
(4) steps for microencapsulation can be controlled with ease,
(5) film walls formed are highly resistive to various solvents,
(6) microcapsules formed have excellent resistance to heat,
(7) microcapsules having desired size and physical strength can be obtained,
(8) the microcapsule slurry obtained has a low viscosity,
(9) the time for the microencapsulating step is short,
and so on.
The process proposed by the foregoing German Pat. No. 2,342,066 is insufficient in solvent proofing property (the above-described requirement (5)), while the processes proposed by U.S. Pat. Nos. 4,233,178, 4,251,386, 4,356,109, 4,219,604 and 4,221,710, and Japanese Patent Application (OPI) No. 51238/81 are insufficient in physical strength (requirement (7)).
Also, a combination of an internal polymerization process with an interfacial polymerization process, as disclosed in British Pat. No. 1,416,224, and the combination of an interfacial polymerization process with an external polymerization process, as disclosed in U.S. Pat. Nos. 4,353,809, 3,981,821, cannot provide such microcapsules as to fulfil both the requirement (5) and (7). When microcapsules insufficient in solvent proofing property are applied to pressure-sensitive recording sheets, a solvent contained in printing ink causes colored fog, or an ingredient incorporated in microcapsules is extracted with a solvent contained in desensitizing ink, a felt-tip pen, or the like, to cause the lowering of color formability.
On the other hand, when microcapsules poor in physical strength corresponding to the requirement (7) are applied to pressure sensitive recording paper, pressure marks tend to occur.